Various photodiode structures are known and the goal in designing these structures depends upon which response characteristics are to be optimized. By way of example, U.S. Pat. No. 5,818,096 in the names of Ishibashi, et al. issued Oct. 6, 1998 entitled Pin Photodiode with Improved Frequency Response and Saturation Output is incorporated herein by reference. Other references in the field related to semiconductor devices are:
S. M. Sze, Semiconductor Devices—Physics and Technology, Section 7.4 on p.283.
Ben. G. Streetman, Solid State Electronic Devices, 3rd edition, Sec.6.3.3 on pp.217–219, with emphasis on FIGS. 6–17.
K. Kato et al., “Design of Ultrawide-Band, High-Responsivity p-i-n Photodetectors”, IEICE Trans. Electron., Vol.E76-C, No.2, pp. 214–221, February 1993.
Kazutoshi Kato, “Ultrawide-Band/High-Frequency Photodetectors”, IEEE Trans. Microwave Theory and Techniques, pp.1265–1281, Vol. 47, Nov. 7, 1999.
S. L. Chuang, Physics of Optoelecironic Devices, Wiley Series in Pure and Applied Optics, John Wiley and Sons, 1995.
J. N. Hollenhorst, “Frequency Response Theory for Multilayer Photodiodes”, Journal of Lightwave Technology, Volume 8, Issue 4, pp. 531–537, 1990.
Jie YAO, K. K. Loi, P. Baret, S.Kwan, M. A. Itzler, “Bandwidth Simulations of 10Gb/S Avalanche Photodiodes”, The 14th Annual Meeting of the IEEE Lasers and Electro-Optics Society, 2001. LEOS 2001. 2, 699–700, 2001.
The pin photodiode taught by Ishibashi is a structure capable of improving the frequency response and the saturation output while maintaining a small RC time constant.
In FIG. 2b of the Ishibashi et al. patent a band diagram of a photodiode in one embodiment has an undoped intrinsic traveling layer serving as a non-absorbing carrier layer. FIG. 9b of the '096 patent is described to be prior art, in contrast with the invention of Ishibashi et al. The only absorption layer within this prior art embodiment of FIG. 9b described and shown by Ishibashi et al., is intrinsic carrier traveling layer, which is light-absorbing.
In contrast to the teaching and invention of Ishibashi et al, and in contrast with the device that he describes as prior art, this invention provides a doped absorption structure for enhanced responsivity x bandwidth by creating at least one of the p-doped or n-doped layers for absorption of light in addition to having a commonly used intrinsic light-absorbing layer. The electrodes are made of non-absorbing high-bandgap material, while the absorption layers are made of low-bandgap material.
The device in accordance with a preferred embodiment, this invention can be viewed as a conventional PIN having n-doped and p-doped absorption layers. In contrast Ishibashi in U.S. Pat. No. 5,818,096 teaches a structure that is absent an n-absorption layer, and, more importantly, is absent an intrinsic absorption, small bandgap layer in accordance with this invention and provides instead an intrinsic non-absorbing (large bandgap) carrier transport layer.
The Ishibashi et al. disclosure teaches a pin diode with high-saturation power; this is at a cost of low responsivity and hence low sensitivity. The design in accordance with this invention provides for high-responsivity and hence high-sensitivity at a cost of lower-saturation power. Hence the structure and response characteristics of the invention described herein are substantially different than the structure described by Ishibashi et al.
The operating characteristics of a conventional pin falls somewhere in between Ishibashi's design and the device in accordance with this invention; notwithstanding, all three designs are high-speed devices. Although the doping in FIGS. 1 and 2 is shown to be uniform, they need not be.
Another prior art reference related to p-i-n photodiodes is U.S. Pat. No. 5,684,308 to Lovejoy et al., who describe a digital photoreceiver comprising a p-i-n photodetector and an adjacent heterojunction bipolar transistor (HBT) formed on the same InP semiconductor substrate, the plurality of constituent InP/InGaAs layers being deposited by a standard epitaxial crystal growth process. The p-i-n photodiode taught by Lovejoy et al. shares the same epitaxial layers as the base and collector of the HBT amplifier. Lovejoy advantageously integrates two devices in the same epitaxial layers; this is highly practicably when the two are similar devices with similar optimization requirements. However, severe compromises must be made when the two devices integrated, in this instance, the p-i-n photodiode and the transistor, are very different and require opposite optimization schemes. For example, with HBT, in order to obtain power amplification from the transistor, the base sheet resistance has to be minimized, and hence the InGaAs base is heavily doped (1e19 cm−3). The heavy doping of the base, in turn, enhances minority carrier recombination, which reduces gain. The heavy doping of the base also reduces carrier mobility, and therefore, reduces the base transit speed. Consequently, the heavily doped base must be very thin to having both the gain and speed. In the embodiment taught by Lovejoy et al, the p-i-n photodiode shares the same epitaxial layers, and hence, the same characteristics are observed. Due to the negligible thickness compared with the collector junction, the heavily doped thin base layer has a minimal effect on enhancing optical responsivity. On the other hand, the very heavy doping in the thin base for example 1e19 cm−3, adversely affects the photodiode speed by reducing the carrier mobility, and adversely affects responsivity by enhancing minority carrier recombination. As a result, the heavily doped thin base InGaAs layer minimally improves the responsivity-bandwidth-product (RBP), if at all.
In contrast, with Lovejoy, the instant invention provides an enhanced responsivity-bandwidth product by ensuring that either the p-doped and/or n-doped absorption layer has a thickness large enough such that the ratio of the thickness of the (p-doped +n-doped absorption layer)/intrinsic layer thickness≧0.17 and preferably larger than 0.20, and/or that the doping concentration dc of one or both of the doped light absorption layers is in between 1e16 and 5e18 cm−3, and wherein the concentration of any doping present in the intrinsic layer is at the most 3e15 cm−3. Compared with Lovejoy et al., the doped absorption layers are thick enough to enhance responsivity, but not very heavily doped so as to keep mobility and speed high and to keep minority carrier recombination low. Under these conditions the doped absorption layers enhance RBP significantly.
In accordance with this invention, the use of doped absorption layers is particularly advantageous for increasing the RBP in very high-speed designs that use a very thin absorber layer. Carrier diffusion in the doped and undepleted absorption layers add negligible transit time, while adding significantly to responsivity, thereby increasing the RBP. The small-depleted portion of doped absorption layer in InGaAs instead of in InP also adds to the total absorber thickness, while keeping unchanged total depletion width, although improvement due to this second mechanism is not highly significant.
Within this specification, the term “intrinsic” shall take its ordinary meaning in the art, which is, undoped or “not intentionally-doped” or “unintentionally doped” (UID). In practice, it is not possible to produce a semiconductor to have absolutely zero dopant atom therein. Hence, the meaning of an “intrinsic” or “undoped” layer is always a UID layer. The UID level is defined by the crystal growth technology at a certain time and at a certain facility. The UID level is lower in more advanced technologies. Since the term intrinsic varies in accordance with semiconductor vendors and with materials, it's common meaning shall be used. Within this specification, the term “doped” in reference to the n-doped and p-doped layers, shall be understood to mean having a higher doping concentration than the intrinsic layer described hereafter. By way of example, the semiconductor material of InGaAs having doping at levels below 1e15 cm−3 is considered to be UID by today's standards.